MESSENGER Set for Final Flyby of Mercury

Sept.
23, 2009: NASA's MESSENGER spacecraft will fly by
Mercury for the third and final time on Sept. 29. MESSENGER
will pass less than 142 miles above the planet's rocky surface
for a final gravity assist required to enter Mercury's orbit
in 2011.

"This
flyby is our final planetary gravity assist, so it is important
for the entire encounter to be executed as planned,"
said Sean Solomon, principal investigator at the Carnegie
Institution in Washington. "As enticing as these flybys
have been for discovering some of Mercury's secrets, they
are the hors d'oeuvres to the mission's main course
-- observing Mercury from orbit for an entire year."

As
the spacecraft approaches Mercury, cameras will photograph
previously unseen terrain, and as the spacecraft departs it
will take high-resolution images of the southern hemisphere.
Scientists expect the spacecraft's imaging system to take
more than 1,500 pictures. So far, more than 90 percent of
the planet's surface has been photographed. These new pictures
will fill in some of the gaps and provide high-resolution
imagery of targets of interest.

"We
are going to collect high resolution, color images of scientifically
interesting targets that we identified from the second flyby,"
said Ralph McNutt, a project scientist at the Johns Hopkins
University Applied Physics Laboratory. "The spectrometer
will make measurements of those targets at the same time."

Above:
A gallery of images from MESSENGER's first two flybys of Mercury.
Highlights include the great Caloris impact basin, the largest
volcano on Mercury, a strangely-elliptical impact scar, and
a fresh crater with spider-like rays. [more]

The
spacecraft may also observe how the planet interacts with
the solar wind. During this encounter, high spectral- and
high spatial-resolution measurements will be taken of Mercury's
super-thin atmosphere and comet-like tail, which may be strongly
influenced by solar activity.

"Scans
of the planet's tail will provide important clues regarding
the processes that maintain Mercury's fascinating atmosphere,"
said Noam Izenberg of the Johns Hopkins Applied Physics Lab.
"The Mercury Atmospheric and Surface Composition Spectrometer
will give us a snapshot of how the distribution of sodium
and calcium in Mercury's atmosphere vary with solar and planetary
conditions. [We also plan to] look for several new atmospheric
constituents."

An
altimeter will make a topographic profile of Mercury's surface
along the instrument ground track. The data will support ongoing
studies of the form and structure of Mercury's craters and
large faults. The information also will extend scientists'
equatorial view of Mercury's global shape and allow them to
confirm the discovery made during the first and second flyby
that Mercury's equatorial region is slightly elliptical.

The
MESSENGER project is the seventh in NASA's Discovery
Program of low-cost, scientifically focused missions.
The Johns Hopkins University Applied Physics Laboratory
of Laurel, Md., designed, built and operates the spacecraft
and manages the mission for NASA's Science Mission Directorate
in Washington. Science instruments were built by the
Applied Physics Laboratory; Goddard; the University
of Michigan in Ann Arbor; and the University of Colorado
in Boulder. GenCorp Aerojet of Sacramento, Calif., and
Composite Optics Inc. of San Diego provided the propulsion
system and composite structure.